This invention will be described in terms of combining an auxiliary (e.g., digital) audio signal with a standard television signal for transmission purposes, however it should be appreciated that the invention is not intended to be limited to this application.
P. R. J. Court, in U.S. Pat. No. 3,231,818 describes a subscription television system wherein the normal audio carrier is encoded with subscription information and program sound is provided on a frequency modulated carrier placed in the vestigial sideband of the television signal. In this instance the frequency modulated carrier is located at 1.0 MHz below the picture carrier.
R. B. Dome, in U.S. Pat. No. 2,635,140 describes a television signal format wherein blue picture signal is modulated on a carrier located in the vestigial sideband of a green picture signal. Dome recognized that the energy of TV signals is concentrated in spectral bands located at integral multiples of the horizontal line frequency. He further recognized that, to effectively separate the signals, it would be advantageous to condition the blue signal to fall between the spectral bands occupied by the green picture signal. Since the blue picture signal is a TV signal, its energy is also concentrated in spectral bands at integral multiples of the line frequency. Locating the blue picture signal between the spectral bands of the green picture signal is accomplished by locating the green picture signal carrier at an odd multiple of one-half the line frequency.
It follows that to effectively separate an auxiliary signal (e.g., a sound signal), combined with a TV signal, the energy of the auxiliary signal should be concentrated between the spectral bands of the TV signal.
However, the energy spectrum of analog sound signals is relatively flat over the frequency band which is typically 20 Hz to 20 kHz. If the sound signal is converted to digital format, e.g., binary, duo binary, etc., the spectral band representing the signal is appreciably broadened (e.g., 350 kHz). It does not occur concentrated in spectral bands located at multiples of the line frequency.
It is an object of the present invention to provide a method and apparatus for conditioning the energy of an auxiliary signal to be concentrated in spectral bands without loss of signal information content.